Introduction to Electricity
What is Electricity? Many years ago, scientists thought that electricity was an invisible fluid. They theorized that this fluid flowed from places which had too much electricity (which they labeled “positive”) to places which had too little (which they labeled “negative”). They were partly right: electricity IS invisible, and it DOES low. but later scientific investigators found that electrical flow is the movement of very tiny particles called ELECTRONS, and that this movement is from negative to positive, instead of vice-versa. They also found that the cause of the movement was not “too much” or “too little” electricity at different places, but the attraction and repulsion of electrical charges. This is the the “electron theory” of electricity.
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The Atom To understand the electron theory, we have to begin with the basic building block of all matter, the atom. ██ The atom resembles a small solar system. • it has a central mass (like the sun). • it has orbiting particles (like the planets). ██ Three kinds of particles are found in the atom. • the central mass, or NUCLEUS, is made up of PROTONS and NEUTRONS. • the orbiting particles are called ELECTRONS. ██ The atoms of each chemical element have different numbers of protons, neutrons and electrons. For example: • a hydrogen atom has one electron, one proton and no neutrons. • a copper atom has 29 electrons, 29 protons and 35 neutrons. ██ In some kinds of atoms, the outermost electrons are lossely bound, and can escape easily. • these “free” electrons move through the material. • large numbers of free electrons make a material a good conductor of electricity.
Electromotive Force
Force must be applied to the free electrons to make them all move in the same direction, if any useful work is to be done. This force is known as “EMF” or “electromotive force.” It is commonly referred to as voltage. ██ The force is generated because particles have different charges. • the electrons have a negative electrical charge. • the protons have a positive electrical charge. ██ Charged paticles behave like small magnets. • particles with UNLIKE charges are attracted to each other. • particles with LIKE charges are repelled from each other.
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A Simple Circuit Electromotive force at work can be illustrated by a simple circuit, like the one shown. The circuit consists of a copper wire connecting the positive and negative terminals of a battery. A small bulb in the circuit lights up to show that electricity is flowing.
██ The copper wire is a good conductor, since copper has many free electrons. • electrons in the wire have a negative charge. They are repelled (pushed away) from the negative terminal
• The negatively charged electrons are attracted by (pulled toward) the positive terminal. This sets up a flow called electronic CURRENT • more electrons must be supplied than are available in the wire itself, or the flow will cease. The battery, or CURRENT SOURCE, supplies the additional electrons. ██ The circuit must be a continuous loop, from the negative terminal to the positive terminal, for current to flow. • if the loop is broken (an open circuit), the flow will cease. ██ Current flow is measured in amperes. • one ampere is equal to more than six billion-billion electrons passing a given point in one second.
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Producing Voltage There are six ways of generating a voltage, the force which moves electrons through a circuit.
(1). Friction
(4). Light
██ When the right types of materials are rubbed together, STATIC ELECTRICITY is produced.
(2). Pressure
██ PHOTOELECTRICITY is the ability of some materials to produce a voltage when light falls on them.
(5). Chemical Action
██ Certain kinds of crystals will produce a voltage when they are squeezed. This is called PIEZOELECTRICITY.
(3). Heat
██
Chemical action between the right materials will produce a voltage. Batteries, which use this principle, are a good portable power source.
(6). Magnetism
██ Voltage will be produced if unlike metals are heated at the point where they are joined. This is THERMOELECTRICITY.
██
Moving a conductor through a magnetic field will produce voltage. This method is used, on a large scale, to generate the electricity supplied by utility companies.
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Volts, Amperes and Ohms Before learning about volts, amperes and ohms, you should become familiar with teh schematic drawings which are used to show electrical circuits. These drawings show al the components of the circuit, and how the components are connected. Coventional symbols are used for various components.
Resistor
Wire
Bulb
-
+
Battery
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Resistance
Current flow through a material is impeded by a property known as resistance. Depending on how much resistance is encountered, current flow is reduced by a greater or lesser amount. ██ Materials with may free electrons offer low resistance. They are CONDUCTORS. Copper is an example. ██ Materials which have few free electrons offer high resistance to current flow. They are INSULATORS. Glass is an example. ██ To limit or reduce the flow of current in a circuit, physical resistors may be added.
Circuit Parameters
Electromotive force, current and resistance are called “circuit parameters.” Each is measurable, and can be given a value. The relationship of the three values determines what is happening in the circuit. ██ Electromotive force, or voltage, is measure in VOLTS. * voltage is represented by V or E (for electromotive force).
██ Current is measured in AMPERES. * the letter I is used to represent amperes in electrical formulas. ██ Resistance is measure in OHMS. *the Greek letter OMEGA (Ω), or an R may be used to represent resistance.
Ohms’s Law
The interrelation ship of the circuit parameters is expressed in the electrical principle known as “Ohm’s Law.” It states that current in an electrical circuit is directly proportional to the applied voltage and inversely proportional to the circuit resitance. ██
If the voltage is doubled, but resistance is kep the same, the current flow in the circuit will double. *EXAMPLE: A circuit has an electromotive force of 5 volts, resistance of 5 ohms, and a current flow of 1 ampere. If voltage is doubled (to 10 volts), but the resistance remains at 5 ohms, the current flow will DOUBLE (to 2 amperes).
██
If the voltage is kept constant and the resistance is doubled, the current flow will be cut in half. *EXAMPLE: A circuit has an electromotive force of 10 volts, a resistance of 5 ohms and a current flow of two amperes. If the voltage remains at 10, and the resistance is doubled (to 10 ohms), the current flow will be halved (to 1 ampere).
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Using Ohm’s Law
If you know the values of any two circuit parameters, you can find the value of the third by using formulas derived from Ohm’s Law. ██ To find CUREENT, divide voltage by resistance
(I=
E R
)
( R=
E I
)
Example: 10 volts ÷ 5 ohms = 2 amperes
██ To find RESISTANCE, divide voltage by current Example: 10 volts ÷ 2 amperes = 5 ohms
██ To find VOLTAGE, multipy current times resistance
( E=IR )
Example: 2 amperes x 5 ohms = 10 volts
Memory Aid
To help remember the different formulas, you can use a memory aid “pie,” like the shown ██ To use the chart, place a finger over the unknown. The reming letters and their positions will tell you what to do. * E over R: divide E by R to find I. * E over I: divid E by I to find R.
E
* I next to R: multiply I by R to find E.
I
R
Memory Aid Pie
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Obeying Ohm’s Law
Three schematics are shown below. The first one shows the values of all three circuit parameters. The second and thrid schematics each have one value missing. Use what you’ve learned about Ohm’s Law to fill in the missing values.
_Ω
5Ω
20V
-
-
-
20V
5Ω
+
+
+
4A
20V
2A
8A
Make Connections
Draw lines to connect each of the circuit parameters at left with the matching measurement unit at right. Electromotive Force
Ampere
Current
Ohm
Resistance
Volt
Review Questions
Complete each of the following questions by writing the correct word or words in the spaces provided. 1. Physical resistors are often inserted in a circuit to __________________ or _________________ current flow. 2. If voltage is held constant and resistance is doubled, current in the circuit will be ______________ If voltage is doubled and resistance is held constant, the current flow will be _________________
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Basic Types of Circuits
There are only three basic types of circuits, so any circuit which you encounter, no matter how complicated it seems, will be on of the three described below: 1. SERIES CIRCUIT ██ This circuit provides only one path for current to follow. * the current is the same in all parts of the circuit * total resistance for the circuit is the sum of all the separate resistances. * the sum of the voltages, measured across each part of the circuit, will equal the source voltage. ██ A problem in one part of the circuit will affect the entire circuit. * a break (open) at any point will cause the current to stop flowing. * a short in one component (such as a lamp) will affect all other components. For example, the remaining lamps will glow more brightly.
R1
SERIES CIRCUIT
R2
E
R3
R5
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R4
2. PARALLEL CIRCUIT ██ This circuit provides several paths for the current to flow in. * branches are connected across (in parallel with) the voltage source. * total resistance of the parallel circuit is less than the resistance of any one branch. * the total current flow in the circuit is the sum of the currents flowing in the branches. ██ A problem in one part of the circuit may not disable the entire circuit.
* an open or short in one branch will have the greatest affect on that branch.
PARALLEL CIRCUIT
E R1
R2
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R3
3. SERIES-PARALLEL CIRCUIT ██ Contains elements of the other two types of circuits. * part of the circuit is in series with the voltage source. * part of the circuit is in parallel with the voltage source ██ A problem in one part of the circuit may or may not disable the entire circuit. * if the problem is in one branch of the parallel portion of the circuit, that branch will be most affected, unless it’s a direct short. * if the problem is in the series portion of the circuit, the entire circuit will be affected. ██ More complex circuits are usually of the series- parallel type.
R1
PARALLEL CIRCUIT
E R2
R3
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R4
Types of Current 1. DIRECT CURRENT (DC) ██ Flows through a circuit in only one direction, from negative to positive. * current from a battery is direct current. 2. ALTERNATING CURRENT (AC) ██ Flows in cycles, or pulses: first in one direction, then in the opposite direction
Alternating Current Cycle * beginning at zero, the current flows in one direction until it reaches a peak, then declines to zero. It then builds to a peak in the opposite direction, and once again declines to zero. * current available from household outlets is alternating current.
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Generating AC Voltage The cycles of alternating curent results from the way it is generated. The generator makes use of the fact that a voltage is produced when a conductor is moved through a magnetic field.
██ With the conductor loop in the position shown in “A”, the voltage is at the zero point. As the loop rotates, voltage increases, reaching its peak at the point shown in “B”. * voltage is at its highest point when the two sides of the loop cut through the magnetic field at right angles. ██
The loop continues to turn, with voltage decreasing from the peak toward zero. It reaches zero when the loop has completed one-half revolution. * during the second half of the loops’ revolution, the voltage will follow the same pattern─building to a peak and dropping back to zero─but the current will flow in the opposite direction.
██ One full revolution of the loop produces one full cycle of alternating current ██ Utility companies generate electricity by using the same principle, only on a much larger scale.
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Effective Value of AC Direct current flows at a consistent value, while alternating current flows in cycles. To compare the two the EFFECTIVE VALUE of the AC must be established. ██ Effective value is the AC voltage which will raise a heating element to the same temperature as a given DC voltage * the effective value of AC is .707 of its peak voltage. Thus, effective voltage is about 7/10 of peak voltage. If the effective voltage is 120, the peak voltage is about 170. * the 120-volt and 240-volt AC values given in everyday usage are EFFECTIVE values. Unless otherwise stated, a voltage can be considered to be an effective voltage.
Draw a Circuit In the space provide, draw an example of a series-parallel circuit. Label the part which is in series with the voltage source, and the part which is in parallel with the voltage source.
AC Generator The two drawings of a model AC generator below show the rotating loop in different positions. One shows the loop at the point where peak voltage is generated; the other the point where zero voltage is being produced. Write “peak” and “zero” under the proper drawings.
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Transformers and Motors Electricity and magnetism are strongly related. Moving a conductor through a magnetic field will produce a voltage. So will moving magnetic field over a stationary conductor. Just as magnetism can produce electricity, an electric current will produce a magnetic field. As current passes through a conductor, a magnetic field is formed around the conductor. That field can be strengthened by coiling the conductor.
Induction (Transformer Action) If the coiled conductor is supplied with alternating current, the magnetic field will follow the current cycles. ██ The first pulse would create a magnetic field in one direction which grows to a peak, then diminishes to zero. ██ The second pluse would cause the field to grow to a peak in the opposite direction, then diminish to zero. ██ By placing another coiled conductor in the magnetic field, an alternating current will be INDUCED, or generated, in that conductor. * this effect is called INDUCTION, or transformer action. * the coil receiving energy from a voltage source is the PRIMARY coil. * the coil which receives its energy from the magnetic field, by means of induction, is the SECONDARY coil.
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Step-up and Step-down Transformers The voltage induced in the secondary coil can be higher or lower than the voltage in the primary coil. Different voltages are achieved by using differing numbers of turns of wire in each coil.
██ If the primary coil has more turns of wire than the secondary coil, the voltage induced in the secondary will be lower than the primary voltage. * this transformer is called a STEP-DOWN transformer, since voltage is decreased in passing through it.
██ If the secondary coil has more turns of wire than the primary, an increased voltage will be induced in the secondary coil. * this transformer is called a STEP-UP transformer, since voltage is decreased in passing through it.
██ Transformers allow AC voltage to be increased for transmission over long distances, then decreased for use in the home.
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Electric Motors The magnetic field generated by current passing through a conductor is also used in the operation of an electric motor. ██ If two coils are formed in the conductor, and arranged as shown, a magnetic field will be set up between the coils. As the cureent alternates, the magnetic field will reverse itself.
██ A bar magnet placed in this field will rotate because of the constantly reversing magnetic field. If the magnet is set on a pivot and spun to start it rotating, it will keep spinning. * As shown, the north and south poles of the bar magnet are being attracted to the opposite poles of the magnetic field Unlike poles are ATTRACTED, or pulled toward each other.
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Electric Motors * As the current alternates, the magnetic field reverses. The poles of the bar magnet and the poles of the magnetic field are now matched: S to S, N to N. Since like poles repel each other the poles of the bar magnet are pushed away, keeping it spinning.
██ The push-pull action caused by the reversing magnetic field continues as long as current is supplied to the coils. It keeps the bar magnet rotating. ██
In most electric motors, the rotating part is a ROTOR cylinder wound with wire or covered with copper strips. It is kept turning in the same way as the bar magnet shown in the example. * To start the rotor turning, some motors have separate start windings. They switch out of the circuit when the motor reaches operating speed.
Measuring Electric Power
The ability of electricity to do work (electric power) is measured in units called WATTS. To find the number of watts of power being produced, voltage is multiplied by current (P=EI). The formula for determining power can be expressed three ways: 1. Power is equal to voltage times current (P=EI) 2. Power is equal to current times current times resistance (P=I 2 R) 3. Power is equal to voltage times voltage divided by resistance (P=
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E2 ) R
Rating Motors Motors are usually rated in horsepower, which is a measure of mechanical energy. Motors are devices which convert electrical energy to mechanical energy. ██ One horsepower is equivalent to 749 watts. ██ In practical terms, a one-horsepower motor will use from 900 to 1200 watts. The additional power is needed to overcome friction and air resistance of the moving parts.
Pick a Pair
A pair of transformers is shown below. Can you pick which one is a step-up transformer and which one is a step-down? To help you, the primary and secondary coils and marked P and S. Write “Step-up” and “stepdown” beneath the proper drawings.
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Watts Up? From the pairs of circuit parameter values given below, determine how many watts of power are being produced. Use the formulas given in “Measuring Electric Power� 1. 500 volts, 50 amperes
=
watts
2. 4 amperes, 5 ohms
=
watts
3. 50 volts, 10 ohms
=
watts
Review Questions
Complete each of the following questions by writing the correct word or words in the spaces provided. 1. The coil of a transformer which receives energy from the ______________ is the secondary coil. In a step- up transformer, the secondary coil will have ______________ turns of wire than the primary coil.
2. One horsepower is equivalent to _______________ watts. Horsepower is the unit of ________________ energy.
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